A cellulose triacetate film has been used conventionally as a base film of silver halide photographic light sensitive materials. The cellulose triacetate film has also been applied to a protective film of a polarizing plate, but an additional property which is not required in silver halide photographic light sensitive materials is required for its use in the protective film. A plasticizer is contained in a cellulose acetate film used for a protective film of a polarizing plate, as in silver halide photographic light sensitive materials, for the purpose of providing flexibility and hydrophobicity to the film.
In recent years, developments have been made to the thinner and light-weight information processing apparatus equipped with a liquid crystal display, such as notebook personal computers, car navigation systems, cell phones and game devices. Requirements for a thinner protective film of a polarizing plate used in a liquid crystal display have become stronger in response to the above-mentioned trend. Therefore, the protective film of a polarizing plate tends to be made extremely thinner, compared to the base film for light sensitive silver halide photographic materials. A decrease of a film thickness results in lowered moisture-resistance, lowering performance of a liquid crystal display in high humidity, specifically under environments of high temperature and humidity.
Further, during the drying process after casting of the film, plasticizers may migrate in the thickness direction of the film web, resulting in inhomogeneous distribution of the plasticizer, and the plasticizer may bleed out onto the web surface or may be evaporated or vaporized to condense and be accumulated on the wall of the film-forming apparatus, forming liquid drops which contaminates the web or rolls. Simply decreasing a cellulose triacetate film thickness deteriorates moisture-resistance, leading to insufficient shield against moisture and deteriorating a polarizer or an adhesive used to adhere the polarizer to the cellulose triacetate film, after an assembly of a polarizing plate. To overcome such a problem, there appeared a proposal to increase the plasticizer in order to compensate for the decrease in film thickness. However, it was proved that a simple increase of the plasticizer accelerated bleeding-out of a plasticizer onto the surface of cellulose film and there took place a new problem in addition to those arising with silver halide photographic light sensitive materials. Additives such as a plasticizer often bleed out or evaporate out of the film under high temperature and high humidity to reduce the weight of the film. The property that such additives are retained within the film is called retainability. Retainability of conventional cellulose acetate films is poor, causing deterioration of performance of a liquid crystal display device.
An increase of a plasticizer further lowers the glass transition temperature (also denoted as Tg) of a cellulose triacetate film, producing problems like softening of the cellulose triacetate film and deteriorated dimensional stability of the film (including shrinkage ratio, coefficient of hygroscopic expansion and coefficient of thermal expansion).
To overcome the foregoing problems, a technique is disclosed, in which a polymer such as polyester, polyester ether, or polyurethane ether, is incorporated as a polymer plasticizer, optionally in combination with a low molecular weight plasticizer, as described, for example, in JP-B 47-760, 43-1635 (the term, JP-B refers to examined and published Japanese Patent), JP-A 5-197073 (the term, JP-A refers to unexamined and published Japanese Patent Application), and U.S. Pat. Nos. 3,054,673, 3,277,031. Further, a technique is also proposed, in which an acryl-type polymer or a polymer obtained by causing an acryl-type monomer to polymerize in the presence of cellulose triacetate is incorporated in a cellulose triacetate film. However, a polymer plasticizer caused phase separation between cellulose triacetate and the polymer in a cellulose triacetate dope, in the web during the drying process or after film formation, resulting in deteriorated transparency, increased water permeability, inhomogeneous shrinkage or lowered shrinkage rate of the film.
A first object of the present invention is to provide a cellulose ester film as a protective film of a polarizing plate which does not contaminate films or equipment because of elution, evaporation or vaporization of low molecular weight additives such as a plasticizer during the film production.
A second object of the present invention is to provide a cellulose ester film as a protective film of a polarizing plate exhibiting superior moisture-resistance, without deteriorating a polarizer even under high temperature or high temperature and high humidity.